Navigational systems which employ a plurality of transmitters transmitting on discrete frequencies are typically used for long range navigation. One such long range navigational system is known as the Omega navigational system and will, when completed, employ eight stations for worldwide surface or airborne navigation. The eight stations each broadcast on three discrete frequencies, the three frequencies being common to each of the eight stations. Omega is a very low frequency (VLF) navigation system operating in the allocated navigation band in the electromagnetic spectrum between 10 and 14 kHz. Global coverage will be obtained with eight transmitting stations.
A hyperbolic Omega receiver measures the phase of two of more Omega stations against a reference generated from an internal oscillator. The internal oscillator permits storage of the phase information so that the relative phases of the different stations can be intercompared. Readout is the phase difference in centicycles between selected stations and ordinarily is recorded continuously on strip chart recorders.
In order that the Omega receiver may determine which station is broadcasting, since all stations broadcast sequentially on the same frequencies, a signal format has been constructed such that each station employs a unique combination of signal duration for each of the three frequencies transmitted. That is, station A transmits the three frequencies sequentially for 0.9, 1.0 and 1.1 seconds, station B transmits for 1.0, 1.1 and 1.2 seconds, station C for 1.2, 1.1 and 1.0 seconds and so forth. By knowing this sequence associated with each station and measuring the duration of the signal, each station can be identified. Each station commences broadcasting its sequence of three frequencies once in every ten seconds. Thus, there is a ten-second delay between the broadcast of the first frequency and the rebroadcast of that frequency from that station. The Omega receiver provides positional information from each of the three frequencies, the positional information being combined into a useable signal for providing an operator with a visual indication of a reference point indicating location.
Basic Omega signals consists of very low frequency 10.2, 13.6 and 11.33 kHz continuous wave pulses. Since there are many requirements associated with transmitting multiple frequencies from multiple stations, an individual discrete frequency, for example 10.2 kHz, is transmitted from an individual station only once in each 10 seconds for a duration of approximately 1 second. Thus, for deriving positional information from a single station, the information is updated with a new signal only once in each 10 seconds. For aircraft moving at a high rate of speed, this interval between updating the position derived from the 10.2 kHz signal may be unsatifactory.
Futher, while each station broadcasts on three discrete frequencies, under adverse atmospheric or ionospheric conditions one or possibly two of the signals may be blanked out. In such event, were the 10.2 kHz signal blanked out for one or two cycles, the update may occur only once in 20 or 30 seconds, an entirely unsatisfactory time interval between corrections to the position derived from the 10.2 kHz input.
It is therefore one feature of this invention to provide a method for improving the reliability and accuracy of multiple frequency receiver navigational systems.
It is a further feature of this invention to maintain tracking even if one or several of the frequency signals from any one station is temporarily lost due to adverse atmospheric conditions.
It is yet another feature of this invention to apply a phase error correction derived from the 10.2 kHz signal to the 13.6 and 111/3 kHz signals as the latter signals are received in sequence following the 10.2 kHz signal.
It is yet another feature of this invention to apply a phase error correction to a plurality of received discrete frequencies, the correction being derived from one sequence of signals and applied to the following sequence of signals either uniformly or with different scaling for each discrete frequency.